Vacuum systems for manufacturing integrated circuits on wafers are generally known. A vacuum processing system may typically have a centralized vacuum chamber, called a transfer chamber, which may be part of a mainframe, for transferring wafers from one process chamber or load lock chamber to the next. A vacuum processing system may also typically have some kind of subsystem, such as a mini-environment, for providing the wafers to the load locks and other chambers and for collecting them back in order to send them on to the next system for processing. This transfer chamber plus the peripheral chambers and staging areas are sometimes called a cluster tool.
Between two vacuum chambers, such as the transfer chamber and one of the process chambers, is a slit valve. The slit valve includes an elongated rectangular opening for providing physical access between the two vacuum chambers. For example, when the slit valve is open, a robot in the transfer chamber may retrieve a wafer from one vacuum chamber and insert it into another vacuum chamber using a long, thin blade to hold the wafer.
After the wafer has been inserted into a vacuum chamber, the slit valve may be closed and sealed with a slit valve door. The slit valve door must form an airtight seal for the slit valve so that the pressure differential between the two chambers will not cause a gas leak through the slit valve. There may also be a metal insert placed within the slit valve opening in order to form a better airtight seat for the slit valve door.
Slit valve doors have typically been made of metal. The metal to metal contact between a slit valve door and the metal insert may provide a very good seal, but a metal to metal contact may create microscopic particles that scrape off of the metal and get into the otherwise relatively clean environment of the vacuum chambers. Such particles may land on the wafers in the chambers, thereby contaminating them. Such contamination is extremely undesirable in the processing of wafers.
To reduce the contamination by particles from the slit valve, an O-ring has typically been placed in a groove in the slit valve door. Thus, a metal to metal contact is avoided, so no particles are thereby generated, and the O-ring provides a satisfactory seal for the slit valve.
Since the seal between the O-ring and the slit valve is not static, but rather is constantly being opened and closed such that there is rubbing and abrading on the O-ring from the slit valve insert, there is still some particle generation, typically from the O-ring. Attempts to stop this particle generation have not been very successful.
Another cause of particle generation from the O-ring has been the rubbing and abrading due to the shape of the groove, or gland, which holds the O-ring in place on the slit valve door. Excessive particle generation, sufficient to render an IC chip defective, at 0.3 microns and smaller have been reported by users. This much particle generation may be sufficient to render an IC chip defective. As seen in FIG. 1, the O-ring 2 has been placed into a dovetail channel 4. The O-ring 2 has a larger diameter than the distance between corners 6, but since it is made of a flexible material, such as rubber, it can be massaged into the gland 4. The O-ring 2 is thus held in place by the sides 8 and the bottom 9 of the gland 4, but in some cases, the O-ring has actually been extracted from the gland during slit valve operation. When the surface of slit valve insert 7 presses against O-ring 2, O-ring 2 is compressed in the direction of arrow A. This compression causes O-ring 2 to expand outwardly against the sides 8. This action causes an abrasion on the O-ring 2 against the sides 8. Additionally, the sharp corners 6 cause even worse abrasion on O-ring 2. All of this abrasion causes particle generation which can contaminate the wafers in the vacuum chambers.
A further problem with the O-rings used in the slit valve doors has been that the O-rings may stick to the slit valve seat, so that when the door opens, the O-ring may be partially pulled out of the dovetail groove, which can damage the O-ring, cause particle generation, and require early servicing of the chamber.
Additionally, the amount of squeeze on the O-ring when metal-to-metal contact occurs will destroy the O-ring in short order due to over-compression and extrusion. The over-compression is a systemic problem that cannot be easily solved. For example, at 80 psig air pressure on a door designed for use with a 200 mm wafer, the force exerted by the 80 mm diameter actuating cylinder is 822 lb. The O-ring seal is designed for normal operation at 15 pounds per linear inch (pli) or 288 lb. of total force. In another example, with the transfer chamber at vacuum, and the process chamber at atmosphere, it takes 464 lb. of actuator force to keep the seal compressed at 15 pli and atmosphere from leaking into the transfer chamber. If this amount of force were to be applied during normal system operation, the O-ring would be severely over compressed.
One way of solving the over-compression problem uses an actuator cylinder with an easily adjustable mechanical stop. The actuator cylinder is the device that closes and opens the slit valve door. A mechanical stop may allow for the compression of the seal to be set within 0.003 to 0.005 inches. This range gives a maximum compression of 520 pounds, or about 27 pounds per linear inch, which is below the upper compressive limits of most seal materials. A mechanical stop on the actuator cylinder, however, does not solve all of the O-ring abrasion and particle generation problems.
It is, therefore, desirable to have a vacuum processing system that provides a slit valve seal that reduces the number of particles generated compared with the prior art, is easy to service or remove, and has a long service life.